MEMS-based micro-hotplates are more and more often used in applications such as miniature flow sensors or gas sensors, the detection principle of which is relying on the temperature elevation of the surrounding gas and/or of an active functional layer. The small size of these micro-sensors offers advantages when integrating in portable devices. However, in such applications not only the small size but also the low power consumption is a key parameter because of its direct consequence on battery lifetime of the portable instrument. For this reason, significant research and development has been carried out, aiming to maximize the efficiency of these sensors, i.e. to maximize sensitivity, selectivity, stability, and speed of response while minimizing the power consumption of the micro-hotplate.
Micro-hotplates are typically based on a membrane structure extending across a rigid frame, on which a resistive heating structure is deposited in order to heat the active area of the membrane to a given desired temperature. The temperature uniformity of the active area is a critical factor for good sensor efficiency, i.e. for an optimum usage of the consumed power which has to be kept as low as possible.
The temperature uniformity can be optimized by using an appropriate pattern for the heating structure.
The most common and straightforward heating resistor pattern used for micro-hotplates is a simple meander of constant width and constantly spaced tracks as illustrated e.g. in EP 0859536 or WO 02/080620. This type of heater represents a nearly uniform heat source but yields a non-uniform temperature distribution due to the conductive heat losses through the membrane to the cold frame. FIG. 1 shows the type of temperature profile obtained across a micro-hotplate by using a uniform heat source (Curve A) compared with the ideally sought uniform temperature profile (Curve B) in the active area (with a target temperature of 500° C. in this example).
In order to correct this lack of temperature uniformity, two strategies have been investigated principally:                to increase the thermal conductivity within the active area by the use of an additional heat spreading plate in order to locally facilitate heat transfer by conduction in the plane of the membrane and thus reduce temperature gradients in the active area;        to locally tune the dissipated heat by varying the heater's section and/or its location.        
The heat spreading plates of the first strategy can be for example silicon islands located underneath the membrane (e.g. WO 00/75649 or “CMOS microhotplate sensor system for operating temperatures up to 500° C.” by Graf et al., Sensors and Actuators B 117 (2006) 346-352) or an additional metal plate located between two dielectric layers between the heater and the active layer (e.g. GB 2464016). This approach reduces more or less efficiently the temperature gradients in the active area, depending on the thermal conductance of the heat spreading plate. However, in any case, the fabrication of this additional heat spreading plate represents a considerable increase in the process complexity, the number of process steps, as well as the risks linked to thermo-mechanical stresses in multiple-layer structures working at high temperatures. Also, this heat spreading plate represents an additional thermal mass that increases the thermal inertia of the micro-hotplate and hence also its thermal response time which is a major drawback for applications where these hotplates are used in cycle mode, e.g. when they are used as IR emitters in IR absorption gas sensors.
The second strategy does not present these drawbacks as it only looks to vary the shape of an already present layer and thus does not require any additional layer, neither any additional process step and also does not increase the thermal mass of the volume to be heated. Various empirical attempts have been made to improve the temperature uniformity of micro-hotplates by local tuning of the dissipated heat. One approach consists in creating one or more hollows in the heater tracks (e.g. EP 1273908, or US 2004/118202) in order to locally, in the hollow, suppress the heat source and in the same time increase the heat dissipation at the borders of the hollow due to the local forced concentration of the current lines. Another approach consists in heating only the periphery of the active area (see for example U.S. Pat. No. 7,279,692 or “Fabrication and characterization of micro-gas sensor for nitrogen oxides gas detection” by Lee et al./Sensors and Actuators B 64 (2000) 31-36). Both of these two first approaches yield typically temperature profiles with a more or less important temperature drop in the non-heated center like illustrated by Curve C in FIG. 1. Other approaches consist in varying the track's section by decreasing progressively the track width from the center to the border of a snail-shaped heater track (EP 2278308) or, to the opposite, by increasing this track width towards the border of the active area (WO 2007/026177). Other designs also combine a variable track width with variable track spacing (“Optimization of a wafer-level process for the fabrication of highly reproducible thin-film MOX sensors” by Elmi et al., Sensors and Actuators B 131 (2008) 548-555). All these very different and even partly contradictory solutions as well as studies like “Design and optimization of a high-temperature silicon micro-hotplate for nanoporous palladium pellistors” (Lee et al. Microelectronics Journal 34 (2003) 115-126) illustrate well the empirical trial and error method used to design these micro-hotplate heaters.
Therefore there are, at the present time, no adequate solutions, for obtaining good temperature uniformity in the active area of the micro-hotplates in order to increase the accuracy, efficiency, and reliability of the sensors using such micro-hotplates.